1. Field of the Invention
The present invention relates to a toner for use in developing electrostatic images formed by a method such as electrophotography. In addition, the present invention also relates to a fixing method for fixing toner images, and an image forming method and a process cartridge in which visual images are produced using a toner.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes:    (1) an electrostatic latent image is formed on an image bearing member such as photoreceptors (electrostatic latent image forming process);    (2) the electrostatic latent image is developed with a developer including a charged toner to form a visual toner image on the image bearing member (developing process);    (3) the toner image is transferred onto a receiving material optionally via an intermediate transfer medium (transfer process); and    (4) the toner image on the receiving material is fixed upon application of heat and/or pressure thereto, resulting in production of a hard copy (fixing process).
Among the methods for use in the fixing process, heat fixing methods are widely used. Recently, in order to save energy, toner images are fixed at a low temperature and therefore the heat energy applied to the toner images becomes smaller and smaller. In addition, it is needed to decrease the warm-up time of image forming apparatus, while the electric power consumption of the apparatus is decreased in a waiting state to protect environment. Requirements for next generation image forming apparatuses are described in the DSM (Demand-side Management) program of IEA (International Energy Agency). There are several requirements therein such that the warm-up time should not be greater than 10 seconds and the power consumption in a waiting state should be not greater than 10 to 30 watt (which changes depending on the copying speed) in copiers having a copy speed not less than 30 cpm (copies per minutes). In order to fulfill these requirements, the power consumption of copiers must be dramatically reduced. In attempting to fulfill the requirements, the following methods have been studied:    (1) to provide a fixing device using a fixing element which has a relatively low thermal capacity and which has a quick temperature rising property while reducing the power consumption in a waiting state; and    (2) to develop a toner having a good low temperature fixability so as to be used for low temperature fixing devices.
Heat fixing methods are broadly classified into contact heat fixing methods and non-contact heat fixing methods. Specific examples of the contact heat fixing methods include heat roller fixing methods, belt fixing methods, etc. Specific examples of the non-contact heat fixing methods include flash fixing methods, oven fixing methods, etc. Among these heat fixing methods, the heat roller fixing methods are widely used because of having the following advantages:    (1) the heat roller fixing methods have good heat efficiency because a heat roller therein is contacted with toner images to be fixed; and    (2) the fixing devices for use in the heat roller fixing methods are small in size.
In attempting to quickly raise the temperature of a heat roller, which results in reduction of the waiting time, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 09-222750 and 2002-82474 have disclosed methods in which a roller having a thickness not greater than 1.0 mm is used as a fixing roller which heats toner images while contacting the toner images. By using such a thin fixing roller, the fixing temperature can be raised to a predetermined temperature in a very short period of time. However, such a heat roller has a low mechanical strength, and therefore a high load cannot be applied to the heat roller to avoid deformation of the heat roller. Since a high load cannot be applied to the heat roller (i.e., since it is impossible for the fixing method using such a thin fixing roller to decrease the lowest fixable temperature while applying a high load to toner images), it is necessary to develop a toner having a relatively low temperature fixability for the fixing method.
In attempting to prepare a toner having a low temperature fixability, a method in which polyester resins, which have relatively good low temperature fixability and good high temperature preservability, are used as binder resins instead of styrene-acrylic resins which have been typically used as binder resins. In order to further decrease the lowest fixable temperature of a toner, it is necessary to control the thermal properties of the binder resin itself of the toner. However, when a resin having a low glass transition temperature (Tg) is used as a binder resin, the high temperature preservability of the resultant toner deteriorates. When a resin having a low softening point [T(F1/2)] is used as a binder resin, the hot offset generating temperature of the resultant toner decreases. Therefore, a toner having a good low temperature fixability and a high hot offset temperature cannot be produced at the present time even when one or more polyester resins are used for the toner while the thermal properties of the resins are controlled.
In attempting to solve the problem, JP-A62-63940discloses a method in which a specific non-olefin type crystalline polymer having a sharp melting property at its glass transition temperature is included in a toner. JP-A2003-167384 discloses a crystalline polyester resin an example of the specific non-olefin type crystalline polymer. However, even when these resins are used, the low temperature fixability of the resultant toners is not fully satisfactory.
Therefore, in order to fulfill the requirements in the DSM Program, a further improved low temperature fixing technique has to be established.